Eliminate the separate types for user space and kernel addresses

[phaubertin]

There are currently two types in the kernel for physical addresses:

• kern_paddr_t for addresses to memory used by the kernel.
• user_paddr_t for addresses to memory used by the kernel.

The original rationale was that the kernel only uses memory under the 4GB, so kern_paddr_t could be 32 bits instead of 64 bits. While this is true, the kernel might still need to map things above this mark like ACPI table.

This PR replaces both types with a single paddr_t type.